Microwave gas tr tubes



APlil 30, 1968 l T. M. NELSON 3,381,167

MICROWAVE GAS TR TUBES Filed Deo. 23, 1965 VEZ' Ej gg 30 32] l l l mlm WIW W22, ze I mii Array/vers- United States Patent O 3,381,167 MICROWAVE GAS TR TUBES Theodore M. Nelson, Baltimore, Md., assigner, by mesne assignments, to the United States of America as represented bythe Secretary of the Air Force Filed Dec. 23, 1965, Ser. No. 516,174 2 Claims. (Cl. 315-39) ABSTRACT F THE DISCLOSURE An electric discharge device having a vacuum tight, gas filled wave guide envelope, resonant elements located within the wa-ve guide envelope, a magnet |applying a constant magnetic iield perpendicular to the RF electric field of the lwave guide envelope and alpha emitting, radioactive coated strips `mounted -Within the wave guide envelope proximate the resonant elements.

This invention relates generally to an electric discharge device in a wfave guide section containing a gas in an enclosure which is capable of -use as a TR tube, and more particularly, to a ydevice having lalpha emitting radioactive material attached to a wall of the Wave guide section, adjacent tips of cone-shaped discharge electrodes extending therefrom, together with a means for producing a magnetic field -for reducing the diffusion rate of the electrons from the discharge area.

Present day electroni-c radar and range finding equipment utilize a common antenna for |`both the transmitting and receiving functions. The TR tube of this invention operates to couple a powerful signal from the transmission element to the antenna and at the same time to protect the crystal of the receiver from the transmission pulses. This short circ'uiting action provides protection for the receiver, and when the receiver is being used, allows for connection of the radar echo signal with the receiver Without a large atten-nation.

The protection of the mixer crystal of the receiver may be performed in accordance with this invention without the use of keep-alive discharge circuitry. To perform this function the immediate breakdown of gas in .a resonant gap 'of the TR tuibe should -be caused by incident RF energy of low power level. In order to insure ionizations at low power levels, a relatively high electron density must be maintained.

Conventionally, this function was performed by keepalive electrodes operated at a suitable potential of about 1000 volts negative at 100 micr-oa-mperes with respect to adjacent parts of the tube. The discharge caused by the keep-alive electrodes prepares the tube -by having a large number of electrons in the gap prior to their appearance of a RF pulse. These conventional keep-'alive systems do not provide protection when the system is olf and when a burst of energy from an adjacent radar system is operating in the vicinity. With aircraft the problem is particulanly acute, land in order to Iavoid power consuming devices tor protection, mechanical shutters have been provided. Becauise of the slow operation `of these devices they have proved to be unreliable.

Accordingly, it is a primary object of this invention to provide a TR tulbe of the `microwave gas type which is completely passive in that it need-s no external circuitry for its proper operation.

It is another olbject of this invention to provide a gas microwave TR tu'be which has bandwidth properties broad enough to protect a receiver mixer crylstal over a broad range of possi-ble incident frequencies.

It is another object of this invention to provide Ia broad bandwidth TR tube which has resonant elements properly spaced within the tube.

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It is still lanother object of this invention to provide a gas microwave TR tube having means for providing residual electrons to provide an electron density sufciently high to correspond to that created by a keep-alive discharge.

It is a further object of this invention to provide a gas microwave TR tube utilizing Iresonant elements in La sealed gas discharge ch'amber wherein electron density is increased by an alpha emitter Kand which has means for providing a constant magnetic field perpendicular to the RF electric lfield of said TR tube in order to reduce breakdown Iby reducing the 4diffusion rate of electrons from the discharge area.

It is another object of this invention to provide a magnetic field for a TR tube to maintain a reduced diffusion rate of electrons from the TR tube discharge area by utilizing a magnet of reasonable size.

These and other advantages, features yand objects of the invention -will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings, wherein:

FIGURE 1 is a view in cross section along lines I-I of FIGURE 2 of one embodiment of this invention;

FIGURE 2 is a cross sectional View along lines II-II of FIGURE l;

FIGURE 3 is a view in cross section along lines IIIIII 0f FIGURE 4 of a partly schematic representation of an alternate embodiment of a TR tube which enables the use of a smaller magnet; and

FIGURE 4 is 'a cross sectional view along lines IV-IV of FIGURE 3.

Referring to FIGURES l an-d 2, there is shown a passive microwave gas TR ltube 10 comprising a wave guide section `11 which is capable of operation with a large X-band copper wave guide. To Iachieve a bandwidth of about 20%, in order to protect a mixer crystal of a receiver utilizing this equipment, four resonant elements 12 of proper Q are provided with a spacing of one qua-rter guide Wave length longitudinally inside the tube and insulated therefrom. Each of the resonant elements 12 comprises a pair of capacitor cones 14 IbetWeen which a discharge is made to occur and a pair of inductive plates 16, as shown most cleanly in FIGURE l. Multiple resonant elements are utilized in order to insure smaller cone gaps wherein the 4center pair of elements have the smallest gaps in order to achieve the greatest bandwidth, as illustra-ted in FIGURE 2. Also, lower breakdown voltages may be realized While keeping the same bandwidth.

Were the TR device to fbe operated in -a narrow band high Q device, only a single resonant element would be required, lrather than the four units shown.

The elimination of a keep-alive discharge circuit is realized by providing residual electrons by means of an alpha emitting radioactive material. In FIGURES` l and 2, these are shown as coated metallic strips 18 incorporated as part of the walls of the wave guide tube. It has been found that high electron density created in a gas by an alpha particle emitter of safe strength is cornparable to that created by a keep-alive discharge. Sufficient alpha emission material is provided to perform this result. Prior art utilization of radioactive materials in TR tubes was provided only to aid in the provision of conventional keep-alive circuits. Thus, in this invention, the alpha emitter material replaces the conventional keepalive discharge in microwave gas TR tubes.

The wave guide section 11 of the TR tube 10 containing the resonant elements and alpha emitter is hermetically sealed at both ends by means of low loss, glass dielectric windows 20 which are joined to flanges 22 forming the corners of the wave guide 11.

The sealed enclosure is evacuated and then filled with the desired quantity of gas. For example, about 1 millimeter Hg of Xenon is preferred because of its low ionization potential and low susceptibility for electron attachment. It should be understood that other gases, such as Krypton, are suitable.

A magnetic field of the proper strength is provided by a permanent magnet 30, and the constant magnetic `field is applied perpendicular to the RF electric field in order to reduce breakdown of the gas. The diffusion rate of the electrons from the discharge gap between the cones 14 of the electrodes 12 is slowed and the electrons couple added energy from the RF electric field. The magnetic field supplied by permanent magnet 30 is placed across the broad walls of the TR tube and centralized at the center cones which have the smallest separation. With a 1.250 inch gap (the distance across the broad walls of the wave guide) a magnetic field, at frequencies of operation near 10,000 kmc., of approximately 3500 gauss is required. A commercial magnet meeting these requirements would be approximately 25 pounds in weight and, therefore, within an usable weight range.

An alternative embodiment which allows for the reduction in weight of the magnet 30 is shown in FIG- URES 3 and 4. Here a half height TR tube 10' is comprised of a wave guide which has end flanges 22', glass dielectric windows 20', and inductive plates 16' as described relative to the embodiment of FIGURES l and 2. Here each of the resonant elements 12' has a single cone 14 which is arranged to discharge to one of the broad walls of the tube. In this embodiment, the radioactive material is applied in strips 18 t0 the inside broad wall just above the cone tips 14 in addition to its position at the base ot` the element 12. By having the material adjacent the cone tips, it provides a density of electrons where it is most needed.

Although the outside wave guide dimensions may be made the same as that of FIGURE 1, by utilizing a double three step transition 24 the device goes from the full height guide at the input and output of the tube to the half height guide. This half height wave guide provides the advantage of exhibiting greater bandwidth than the full height guide and it puts the same RF power into a smaller cross sectional area to further aid in reducing breakdown.

The magnet 30' in this embodiment is positioned atop the broad wall of the tube and is centrally located in the transverse direction at the cone 14 whose tip is closest to the wall. Pole pieces 32 are provided in the wall to help draw the magnetic field inside the tube. Thus, the fringing field of the magnet is used and disturbance of the magnetic field is avoided by the elimination of other magnetic material near the tube. This situation is also the case with the embodiment of FIGURES 1 and 2. rI `he magnetic arrangement of the embodiment of FIGURES 3 and 4 creates lower breakdown in microwave gaseous switching while utilizing a reasonable sized magnet.

The full height tube 10 of FIGURE 1 and the half height tube 10 of FIGURES 3 and 4 provide protection against burnout of radar crystals. The combination of appropriately placed radioactive strips in conjunction with the permanent magnet, when applied to a TR tube having discharge elements, results in the provision of a superior protective device,

Although the invention has been described with reference to particular embodiments, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

1. An electric discharge device capable of being operated as a transmit-receive tube comprising a vacuum tight, rectangular, gas filled wave guide envelope having top and bottom broad walls, a transition means in said wave guide envelope mounted on said bottom broad wall forming a half height wave guide envelope at the center thereof, resonant elements in said wave guide envelope mounted on said transition means, said elements comprising at least one capacitive cone-shaped element and at least a pair of induction plates wherein said cone-shaped element is arranged to discharge to said top broad wall of said wave guide envelope, alpha emitting, radioactive coated strips mounted on the inside of the top broad wall of said wave guide envelope just above said cone-shaped element and mounted adjacent the base of said coneshaped element on said transition means providing a density of electrons where it is most needed for low power ionization, means to apply a constant magnetic eld perpendicular to the RF electric field of said wave guide envelope, said last-mentioned means located atop the top broad wall of said wave guide envelope and having pole pieces provided in said top broad wall.

2. An electric discharge device as defined in claim 1 having a series of said resonant elements with quarter wave length spacing.

References Cited UNITED STATES PATENTS 2,602,908 7/1952 Linder 333-13 XR 2,652,618 9/ 1953 Prescott 315-39 XR 2,697,800 12/1954 Roberts 315-39 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

M. L. NUSSBAUM, Assistant Examiner. 

